The present invention relates to a method of manufacturing golf balls in which a specific thermoplastic polyurethane composition is used in the cover. More specifically, the invention relates to a method of manufacturing golf balls in which a thermoplastic polyurethane composition that is recyclable in a molding operation is used in the cover, which golf balls have a high rebound, excellent spin characteristics and scuff resistance, and an excellent manufacturability.
The use of polyurethane materials as golf ball cover materials has received some attention in recent years. Polyurethane materials are broadly divided, based on the process used to make molded articles therefrom, into thermoset polyurethane materials and thermoplastic polyurethane materials. Molded articles made of thermoset polyurethane materials can be obtained by mixing under applied heat a urethane prepolymer having isocyanate end groups with a liquid starting material such as a polyol or a polyamine as the curing agent, then pouring the mixture directly into a mold and heating to effect a urethane curing reaction.
Numerous golf balls which use such a thermoset polyurethane material have been disclosed in the prior art (e.g., Patent Document 1: U.S. Pat. No. 5,334,673, Patent Document 2: U.S. Pat. No. 6,117,024, and Patent Document 3: U.S. Pat. No. 6,190,268). Methods of molding thermoset polyurethane materials are described in, for example, Patent Document 4: U.S. Pat. No. 5,006,297, Patent Document 5: U.S. Pat. No. 5,733,428, Patent Document 6: U.S. Pat. No. 5,888,437, Patent Document 7: U.S. Pat. No. 5,897,884, and Patent Document 8: U.S. Pat. No. 5,947,843.
Because moldings made of thermoset polyurethane materials lack plasticity when heated, the starting materials and molded articles cannot be recycled. Moreover, given the length of the heating and curing step and of the cooling step and given the difficulty of controlling the molding time owing to the high reactivity under heating and the instability of the starting materials, the manufacturability of specialty moldings such as golf ball covers (moldings which enclose a core material) made of thermoset polyurethane materials is regarded as inefficient.
By contrast, moldings made of thermoplastic polyurethane materials are not obtained by directly reacting the starting materials. Instead, a linear polyurethane material synthesized using starting materials and a production method which differ somewhat from those for the thermoset polyurethane materials described above is employed in the molding operation. Such a polyurethane material is thermoplastic, and thermoplasticized polyurethane materials are of a nature as to solidify on cooling. Such polyurethane materials can thus be molded using an injection molding machine. The injection molding of thermoplastic polyurethane materials requires a much shorter molding time than thermoset polyurethane materials and moreover is suitable for precision molding, making it ideal as a process for molding golf ball covers. In addition, thermoplastic polyurethane materials are recyclable and are friendly to the global environment. Golf balls made using thermoplastic polyurethane materials are disclosed in, for example, Patent Document 9: U.S. Pat. No. 3,395,109, Patent Document 10: U.S. Pat. No. 4,248,432 and Patent Document 11: U.S. Pat. No. 4,442,282.
However, prior-art golf ball covers made with thermoplastic polyurethane materials have been unable to satisfy at the same time requirements for feel on impact, controllability, rebound and scuff resistance when hit with an iron.
To address this need, Patent Document 12: JP-A 9-271538 discloses a golf ball cover made using a high-resilience thermoplastic polyurethane material. Yet, even this golf ball cover falls short in terms of its scuff resistance when hit with an iron.
Patent Document 13: JP-A 11-178949 describes a golf ball cover which has a relatively good scuff resistance when hit with an iron and is composed primarily of the reaction product of a thermoplastic polyurethane material with an isocyanate compound. In this cover, an isocyanate compound such as a diisocyanate or a block isocyanate dimer is added as an additive to the thermoplastic polyurethane material. Addition is carried out during melt mixing under applied heat using an extruder or during injection molding, with the reaction being effected during molding.
However, in the molding of a cover according to JP-A 11-178949 above, the isocyanate compound is hard to handle because it loses its activity in the presence of moisture, thus making it difficult to obtain a stable reaction product. In the case of blocked isocyanates, which are highly resistant to moisture absorption, the blocking agent that dissociates under heating has a strong odor, making it unsuitable for use in molding covers. Moreover, when the isocyanate compound is in the form of a powder or a solution, it is difficult to control the amount of addition to the thermoplastic polyurethane material, making control of the golf ball cover properties a challenge. Furthermore, owing to melting point and melt viscosity differences between the thermoplastic polyurethane material and the isocyanate compound, slippage arises within the molding machine, which sometimes makes thorough kneading impossible to achieve. In this prior art, for the reasons given above, control of the effects of moisture within the cover material and of the amount of additive included therein has been inadequate, making it impossible to achieve golf ball covers which are fully satisfactory in terms of their scuff resistance-improving effects.
The preferred thermoplastic polyurethane material described in JP-A 11-178949 is based on an aliphatic isocyanate. However, this thermoplastic polyurethane material has a very large reactivity with isocyanate, making the reaction difficult to control. As a result, one problem is that gelation tends to arise before the mixture is used in injection molding, making it impossible to ensure sufficient plasticity. Another problem is that gelation sometimes occurs during the molding operation. Yet another problem is that the resin being recycled sometimes gels, as a result of which reclamation becomes impossible. These problems have made it difficult to put the above technology to practical use.
Patent Document 14: JP-B 58-2063 (U.S. Pat. No. 4,347,338) describes a method of manufacturing thermoset polyurethane molded articles which involves intimately mixing a compound bearing two or more isocyanate groups with a thermoplastic resin which does not react with isocyanate groups, blending the resulting mixture with a thermoplastic polyurethane material, then furnishing the blend to a molding machine and molding. However, the objects of this prior art are simply to improve the resistance to solvents and to continuous and repeated friction; the published specification does not indicate the use of this prior-art molding material as a golf ball cover material. Accordingly, there continues to exist a desire for a golf ball cover material which can satisfy a number of properties required of golf balls; i.e., rebound, distance, spin characteristics, controllability, feel on impact, scuff resistance, cut resistance, and discoloration resistance.
Patent Document 15: JP-A 2002-336378 discloses a golf ball which uses a cover stock composed of a thermoplastic polyurethane material and an isocyanate mixture. This cover stock is a thermoplastic polyurethane material which is recyclable and has a high rebound and an excellent scuff resistance. Although such cover stock exhibits both the good manufacturability of a thermoplastic polyurethane and physical properties on a par with those of thermoset polyurethanes, it also has a number of drawbacks, including the formation of scorched material when the isocyanate mixture is charged into the molding machine and a poor molding stability.